In a disc grinder used for cutting, grinding, and/or performing other operations on stone, concrete, or the like, switching a start switch to the ON side (ON position) starts an electric motor and thereby rotates a circular grinding wheel. The start switch is configured such that it can be locked (a so-called “lock-ON” function or feature) in the actuated or ON state so that the user is not required to continuously hold the start switch in the ON position while using the grinder to process a workpiece, thereby facilitating the processing work. Japanese Laid-open Patent Publication 2012-76160 and its English counterpart US 2013/0186661 disclose a technique in which the lock-ON function of the start switch is implemented by microcontroller control (motor control), which controls the actuation of the electric motor serving as a drive source.